1. Field of the Invention
The present invention is related to apparatus for cooling samples and more particularly to cooling systems that use expendable evaporating coolants.
2. Background of the Invention
Sample measurements, such as those conducted for thermal analysis experiments, frequently involve performing measurements at temperatures that are below the ambient temperature of a measuring instrument used to conduct the experiment. This requires that at least a portion of the instrument be cooled, such as a sample stage region used to hold the sample. The sample stage region is cooled by providing a heat exchange means (heat exchanger) in a region adjacent and thermally coupled to it or as part of the region that holds the sample. Coolant is then provided to the heat exchanger to remove heat from the sample region.
Generally, cooling systems designed for such purposes may be divided into those that use expendable coolants like liquid nitrogen, and those that recirculate a coolant, such as those that use vapor-compression refrigeration. In cases in which liquid nitrogen is used as a coolant, there are broadly two different approaches, which may be divided into (1) those that cool a sample stage by using cold vapor to cool the apparatus by convective heat transfer and (2) those that use liquid nitrogen to cool the apparatus by boiling heat transfer. Systems using convection cooling are relatively inefficient because the latent heat of vaporization, which has by far the greater cooling effect per unit mass of coolant, is not used to cool the apparatus. Also, considerably lower temperatures may be achieved using boiling heat transfer as compared to convective cooling. Using convection cooling it is often difficult to reach −150° C., while systems that use boiling heat transfer cooling can readily achieve −180° C. without any particular difficulty.
Often, the choice of convection heat transfer using vapor rather than boiling heat transfer is made based on the method of temperature control of the instrument, and not based on considerations of maximizing cooling efficiency. This is due to differences in the nature of the two heat exchange processes. When using convection cooling, heat transfer between the vapor and a heat exchanger depends directly upon the flow rate of the vapor. At low flow rates, the temperature change experienced by the vapor is large and the cooling effect is low, while at high flow rates, the temperature change experienced by the vapor is lower, but the cooling effect is much greater; thus, one may regulate the cooling effect by changing the flow rate of the vapor. In the boiling heat transfer process, the liquid in a heat exchanger is always essentially at the boiling point of the saturated liquid and the vapor that is created from the liquid is also very close to the boiling point. Thus, increases in flow rate have very little effect on the magnitude of heat exchange, so that adjusting the flow rate of the liquid does not appreciably change the amount of heat removed, as long as the volume of boiling liquid in the heat exchanger remains constant.
Boiling heat exchange involves the formation of vapor bubbles on a surface of the heat exchanger, wherein the bubbles detach from the surface and carry away heat in the process. As the heat flux across the heat exchange surface increases, the rate of bubble formation increases. Eventually, if the rate of heat exchange continues to increase, the surface becomes almost completely covered with vapor bubbles and the rate of heat exchange reaches a maximum. At that point, the critical heat flux (also termed “critical heat flux point” or “critical heat flux level” hereinafter) is reached, and further increases in heat flux cause the temperature of the heat exchanger to rise rapidly, as the rate of heat flux across the surface drops. The critical heat flux is an unstable point, above which the heat flux across the surface drops as the surface temperature rises. To avoid the instability associated with reaching the critical heat flux, cooling systems that use boiling heat exchange are designed to always operate below the point of critical heat flux, which requires that a boiling heat exchange cooling system always operates with a constant level of liquid in the heat exchanger. It is therefore desirable that the heat exchanger is designed to have enough surface area such that the critical heat flux is not exceeded when maximum power is developed in the instrument being cooled. It is also desirable that a pump used to deliver liquid to the heat exchanger is designed to deliver more liquid than necessary to replenish liquid that boils during the heat exchange process, including transfer losses of liquid, i.e., liquid that boils in the transfer line between the pump and the heat exchanger. Thus, for an instrument that uses boiling heat transfer cooling, during an experiment it is paramount that the liquid level in the heat exchanger remains constant to maintain the heat exchanger in a stable boiling regime below the critical heat flux. Regulation of the flow rate of coolant is of lesser concern.
Known systems that employ a dewar as a source of liquid nitrogen coolant (see, for example U.S. Pat. No. 6,578,367 to Schaefer, et al.; U.S. Pat. No. 5,117,639 to Take; U.S. Pat. No. 5,013,159 to Nakamura, et al.; U.S. Pat. No. 4,979,896 to Kinoshita; U.S. Pat. No. 4,783,174 to Gmelin, et al.; U.S. Pat. No. 4,031,740 to Achermann; U.S. Pat. No. 3,572,084 to May; U.S. Pat. No. 3,456,490 to Stone, which are all incorporated by reference herein in their entirety, and see J. G. Van-de Velde, J. D. Mitchell, Thermochimica Acta, 214 (1993) 163-170) are configured to pressurize the dewar to transfer the nitrogen coolant, in either liquid or gas form, to the heat exchanger during an experiment. In most cases, the dewar is pressurized by using nitrogen gas evolved during evaporation of the liquid in the dewar. Two variations are generally used: 1) a heater is immersed in the liquid nitrogen and is used to boil a portion of the liquid, such that the expansion of the generated vapor pressurizes the dewar; and 2) a tube connecting the bottom of the dewar to the gas space at the top of the dewar is passed between the inner and outer walls of the dewar, such that liquid flowing into the tube boils. The resulting vapor enters the top of the dewar, thereby pressurizing the dewar.
When a pressurized dewar is used to transfer liquid to a heat exchanger, the gas pressure in the dewar forces the liquid to enter a transfer tube that extends downward into the dewar, terminating just above the bottom of the dewar. The liquid flows upward through the transfer tube by which the liquid is conducted to the sample region thereby providing a cooling medium to cool the sample during an experiment. When the pressurized dewar is used to transfer gas, the transfer tube is connected to the gas space above the liquid in the dewar and the pressure in the dewar forces the cold gas to flow through the transfer tube to the experiment. In either case, refilling the dewar with liquid nitrogen is required periodically to replenish the liquid nitrogen lost to the gas phase during an experiment. Refilling the dewar requires that it be vented to atmosphere to allow vapor generated during filling and displaced by the liquid to be discharged to atmosphere. Thus, the pressure necessary for delivering coolant to a heat exchanger located near the sample during the experiment must be released each time the dewar is refilled.
When the pressure from the dewar is released, the desired cooling effect at the heat exchanger is disrupted. For example, in a boiling heat exchange system, as mentioned above, it is paramount that the level of liquid be maintained at a certain level to avoid heat exchange instability. Once the transfer line conducting liquid to the sample heat exchanger is depressurized due to depressurization of the dewar, the circulation of the liquid through the heat exchanger slows down or stops, and the liquid in the heat exchanger boils, reducing the liquid level, potentially causing the critical heat flux to be exceeded which destroys the ability to control the sample temperature as desired. Because of this problem, an experiment must generally be terminated for refilling of a pressurized dewar. This limits the maximum duration of experiments that may be performed using known pressurized dewar systems and/or requires that the experimental schedule allow for refilling the dewar.